Explanation:

**Dyarchy** literally means 'rule of two' or 'double government'. In the context of British India, it referred to a system of governance introduced by the Montagu-Chelmsford Reforms (Government of India Act 1919) in the provinces. Under dyarchy, the executive authority was divided into two parts: 'reserved subjects' (like finance, law and order, land revenue) which remained under the control of the Governor and his Executive Council (who were not responsible to the legislature), and 'transferred subjects' (like education, health, local self-government) which were administered by the Governor with the aid of ministers responsible to the provincial legislative council. Therefore, Option D, 'A government in which power is vested in two rulers' (or two distinct sets of authorities), is the correct definition. Option A describes imperialism or colonialism. Option B describes a dictatorship. Option C describes an oligarchy or aristocracy. None of these accurately define dyarchy as it was implemented in India.